Hi,
I sell internationally and have no problems.
I use Stamps.com to buy my stamps (other than using the Ebay postage).
When I ship, I use USPS Internationl 1st Class Mail and ship it.
I haven't had any problems with the buyer receiving the decals nor with customs. However, I do NOT ship to Africa and South America. Too many bad reports from other sellers of customs delays and buyers not receiving their products.
Tips: if it's just vinyl decals/lettering, just pack it in a clear plastic bag and ship it. If you have lettering or decals that are large or the lettering is longer where it has to be either folded or rolled, you might want to try a round cardboard tube or (for lettering that is long), roll it and place it in a suitable cardboard box. This would prevent any damage.
Bear in mind that when you ship overseas, the rates are high. When figuring postage in your Etsy listing, double the actual weight and don't forget to include the box, tube, envelope.
Hope this helps.
Fred
Melbourne FL